CONDENSED CONSOLIDATED INCOME STATEMENTS - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Operating revenues	$ 1,608.0	$ 1,643.7	$ 5,575.6	$ 5,602.7
Operating expenses
Cost of sales	484.3	524.1	1,985.8	2,043.9
Other operation and maintenance	529.2	553.1	1,583.4	1,602.7
Depreciation and amortization	233.8	212.8	690.1	628.1
Property and revenue taxes	49.8	51.0	148.0	149.4
Total operating expenses	1,297.1	1,341.0	4,407.3	4,424.1
Operating income	310.9	302.7	1,168.3	1,178.6
Equity in earnings of transmission affiliates	38.7	33.7	111.7	95.2
Other income, net	21.8	26.1	76.3	65.0
Interest expense	125.8	112.0	374.3	327.2
Other expense	(65.3)	(52.2)	(186.3)	(167.0)
Income before income taxes	245.6	250.5	982.0	1,011.6
Income tax expense	11.3	17.0	91.5	156.4
Net income	234.3	233.5	890.5	855.2
Preferred stock dividends of subsidiary	0.3	0.3	0.9	0.9
Net loss attributed to noncontrolling interests	0.3	0.0	0.5	0.0
Net income attributed to common shareholders	$ 234.3	$ 233.2	$ 890.1	$ 854.3
Earnings per share
Basic (in dollars per share)	$ 0.74	$ 0.74	$ 2.82	$ 2.71
Diluted (in dollars per share)	$ 0.74	$ 0.74	$ 2.81	$ 2.70
Weighted average common shares outstanding
Basic (in shares)	315.4	315.5	315.4	315.5
Diluted (in shares)	316.8	316.9	316.7	316.9